In general, a reactor has, for example, a winding and a core made of a magnetic substance and the winding is wound around the core to make up the coil of the reactor, which enables inductance to be obtained. Conventionally, the reactor is used in a voltage boosting circuit, inverter circuit, active filter circuit, or the like, and, in many cases, such the reactor has a structure in which the core and the coil wound around the core are housed, together with other insulating members or the like in a case made of metal or the like (see, for example, Patent Reference 1). Further, for example, in a reactor to be used in a vehicle-mounted voltage boosting circuit, a coil is used which has a structure in which two single-coil elements each having a predetermined winding diameter and the number of windings that can provide a high inductance value in a high current region are formed in parallel to each other and are coupled (connected) to each other so that the directions of currents flowing through both the coils are reversed to one another (see, for example, Patent Reference 2).    Patent Reference 1: Japanese Patent Laid Open Publication No. 2003-124039    Patent Reference 2: Japanese Patent No. 3737461